I will not say anyting. I will just continue to walk like I dont know her/him and when she calls I will say "I am sorry, I know a man/woman with your name" I'll just erase her from my life and thats it. Let her be happy with that other. The decision was taken by her when she chose the other man and thats it. Over.

well unlucky me .. cant talk on this topic as i dnt have ne g/f .. lolzz kidin.. hmm first i will let her explain me the reason and if its true and reliable then may be forgive her, oda wise u kno......

If I saw this I would never speak to my boyfriend again. I think it is unforgivable for them to have an affair with someone else.
Of course if I wasn't sure whether they were just being friendly or if it was more I would wait a bit and see if I could find out.

I will make it known that I have caught them, and then walk away and never speak to them again. You have to make a point to make them feel guilty, and then they feel even more guilty through the use of the silent treatment. Thats just what I would do, but I dont know about anybody else.